a) Field of the Invention
The present invention relates to a real image type view finder.
b) Description of the Prior Art
As conventional examples of real image view finder which is compact and has a high vari-focal ratio, there are known view finders each of which consist, in order from the object side, of a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power and a fourth lens unit having positive refractive power, and is configured to change a magnification by moving the second lens unit and the third lens unit along an optical axis, like view finders disclosed, for example, by Japanese Patents Kokai Publication No. Hei 6-300971, No. Hei 7-13076 and No. Hei 5-93863.
These conventional real image type view finders uses objective optical systems having short back focal lengths and eyepiece optical systems using short prisms, thereby being low in prism composition freedom.
Out of the above described conventional examples, view finders according to first through third embodiments of the Japanese Patent Kokai Publication No. Hei 7-13076 have low vari-focal ratios, and view finders according to fourth and fifth embodiments of this patent have high vari-focal ratios but produce remarkable longitudinal chromatic aberration at tele positions, whereby it cannot be said that these view finders have sufficiently favorable optical performance.
Furthermore, the conventional example of view finder disclosed by Japanese Patent Kokai Publication No. Hei 6-300971 uses the second lens unit which consists of two lens elements, thereby having a defect of relatively high cost
Furthermore, the conventional view finder disclosed by Japanese patent Kokai Publication No. Hei 5 -93863 also has a low vari-focal ratio.
Furthermore, a conventional example disclosed by Japanese Patent Kokai Publication No. 2000-39649 is a real image type view finder which has a composition shown in FIG. 1 and is suited to configure a camera compact. This real image type view finder used an objective optical system which consists of positive, negative, negative and positive lens elements, but the Publication makes no disclosure of a composition which satisfies conditions for both favorable optical performance, and a compact and thin configuration.
An object of the present invention is to provide a real image type view finder comprising an objective optical system which has positive refractive power and forms an intermediate image, an image erecting optical system which comprises at least a prism and erects the intermediate image, and an eyepiece optical system which leads the intermediate image to an observer""s eye, wherein the objective optical system comprises, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power and a fourth lens unit having positive refractive power, changes a magnification by moving the second lens unit and the third lens unit, and satisfies the following condition (1):
0.97 less than Lr/S less than 1.2xe2x80x83xe2x80x83(1) 
A second object of the present invention is to provide a real image type view finder comprising an objective optical system which has positive refractive power and forms an intermediate image, an image erecting optical system which comprises at least a prism and erects the intermediate image, and an eyepiece optical system which leads the intermediate image to an observer""s eye, wherein the view finder comprises an optical member disposed in the vicinity of the intermediate image separately from the prism and is configured to allow the optical member to be exchanged independently.
A third object of the present invention is to provide a real image type view finder comprising an objective optical system which has positive refractive power and forms an intermediate image, an image erecting optical system which comprises at least a prism and erects the intermediate image, and an eyepiece optical system which leads the intermediate image to an observer""s eye, wherein the objective optical system comprises, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power and a fourth lens unit having positive refractive power, changes a magnification by moving the second lens unit and the third lens unit in an identical direction along an optical axis, and satisfies the following condition (2):
xe2x88x920.4 less than f4(f3xc3x97z) less than xe2x88x920.185 xe2x80x83xe2x80x83(2) 
A fourth object of the present invention is to provide a real image type view finder comprising an objective optical system which has positive refractive power and form an intermediate image, an image erecting optical system which comprises at least a prism and erects the intermediate image, and an eyepiece optical system which leads the intermediate image to an observer""s eye, wherein the objective optical system comprises, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power and a fourth lens unit having positive refractive power, changes a magnification by moving the second lens unit and the third lens unit in an identical direction, and satisfies the following condition (6):
1.1 less than (f1xc3x97f2)/(f3xc3x97f4) less than 1.6 xe2x80x83xe2x80x83(6) 
A fifth object of the present invention is to provide a real image type view finder comprising an objective optical system which has positive refractive power and forms an intermediate image, an image erecting optical system which erects the intermediate image, and an eyepiece optical system which leads the intermediate image to an observer""s eye, changes a magnification by moving the second lens unit and the third lens unit, and satisfies the following condition (9):
0.8 less than (L3xe2x88x92L2)xc3x972/L2 less than 5.5xe2x80x83xe2x80x83(9) 
A sixth object of the present invention is to provide a real image type view finder comprising an objective optical system which is configured separately from a photographic optical system and forms an intermediate image, a visual field frame disposed in the vicinity of the intermediate image, and an image erecting optical system which erects the intermediate image, wherein the objective optical system comprises, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power and a fourth lens unit having positive refractive power, a focal length of the objective optical system can be shorter than that of an eyepiece optical system, and the view finder satisfies the following condition (7):
0.53 less than H (max)/fe less than 1xe2x80x83xe2x80x83(7) 